1. Field of the Invention
The present invention is related to the field of wastegated turbomachinery such as turbochargers. More specifically, the invention relates to such wastegated turbochargers where the bypassed exhaust gas is returned to the main exhaust flow through an ejector nozzle.
2. Description of Related Art
Turbochargers have been commonly applied to internal combustion engines and their benefits of improving engine performance and reducing emissions is generally known in the art. In such applications, the exhaust gas of the internal combustion engine is routed to a turbine wheel where the energy of the exhaust gas is used to rotate the turbine wheel within its housing. The turbine wheel is fixedly connected to a compressor by a shaft causing the compressor to rotate and compress the intake charge. This compressed charge is routed to the intake manifold of the engine and thus, is provided to the engine cylinders during the intake cycle. The compressed intake charge improves the performance and the efficiency of the engine. Turbochargers are also generally provided with a wastegate which diverts at least a portion of the exhaust gas thereby providing a bypassed flow which circumvents the turbine wheel. Thus, the wastegate limits the amount of exhaust gas directed to the turbine wheel thereby controlling turbine wheel over-speed and the maximum boost pressure provided by the compressor. In conventional wastegated turbocharger designs, the bypassed flow generally passes through a poppet valve located just upstream of the turbine wheel and re-enters the main exhaust flow immediately downstream of the turbine wheel.
It has been found that this re-entry of the bypassed flow into the main exhaust flow in conventional wastegate designs diminishes the engine's theoretical efficiency in two distinct ways. First, the bypassed flow possesses wasted energy which is not harnessed or utilized by the turbocharger. Secondly, the main exhaust flow exiting the turbine wheel is disrupted when the bypassed flow re-enters the main exhaust flow. This disruption increases the turbine back pressure and the engine's back pressure which correspondingly reduces the efficiency of the turbocharger thereby diminishing the efficiency of the engine.
Attempts have been made to address the above identified problem of flow disruption and the resulting increased back pressure. In minimizing the main flow disruption, it has been also recognized in the art that in theory, the re-entering bypassed gas may be used to actually reduce the engine back pressure thereby increasing the engine's performance and efficiency. More specifically, it has been suggested in the art that this reduction in engine back pressure can be accomplished by having the bypassed flow re-enter the main exhaust flow through an ejector. Such flow would create an ejector effect which is a localized region of decreased pressure. This ejector effect will, in essence, reduce the pressure of the main exhaust flow when the bypassed flow re-enters the main flow and consequently, reduce the back pressure on the engine. Such reduction in back pressure will increase the turbocharger's performance and efficiency thereby increasing the performance and efficiency of the engine. Furthermore, some prior art references have specifically proposed various ejector designs that may be used in attempting to attain the desired ejector effect when the bypassed flow re-enters the main turbine exhaust flow.
For example, one reference that discusses the ejector effect of bypassed flow in a turbocharger is exemplified in U.S. Pat. No. 4,463,564 to McInerney that discloses a turbocharger with a bypass passage in the turbine housing and a wastegate which provides an exhaust ejector for reducing back-pressure on the engine. More specifically, the McInerney reference discloses a swing-type wastegate valve and bypass passage configured to inject the bypassed exhaust gases angularly and at a relatively high velocity into the exhaust passage such that the bypassed flow tends to draw gases from the turbine wheel thereby reducing back pressure. However, efficient ejectors are very difficult to design and consequently, it has been found that no significant ejector effect is likely to occur with simple bypass passages such as those disclosed in McInerney.
Alternative ejector designs are also disclosed in the Japanese Patent No. 55-25505 to Sato et al. which shows a turbocharger where the bypassed flow is jetted from an annular slit provided downstream of the turbine. The reference further discloses that the outlet static pressure of the turbine is thereby reduced and the output of the engine is increased. A similar ejector design is disclosed in the European Patent No. 0034765 to Behnert et al. which shows a turbocharger with an annular chamber formed around the turbine outlet such that the bypassed flow re-enters the main exhaust gas flow from the annular chamber with a swirling motion. This reference postulates that the swirling motion reduces the static pressure which results in an increase in the turbine speed. Another alternative design is shown in International patent application WO 89/07194 to Baretzky which discloses a turbine with a bypass channel including outlet openings with a radial slot shape which open into a turbine outlet in the direction of the exhaust gases flowing through the turbine outlet and tangent to the direction of turbine rotation. The reference discloses that the bypassed flow exerts an ejector effect on the exhaust gases, thereby reducing the pressure behind the turbine.
An ejector design including an annular ring jet similar to those discussed above has been tested by Aisin Seiki as disclosed in SAE paper "Development of SJ (Swirl Jet) Turbocharger for Diesel Engine Vehicles" (1997). The reference discloses a wastegated turbocharger in which the bypassed flow re-enters the main exhaust gas flow through an annular "swirl jet" which induces a swirling effect on the re-entering bypassed flow. The reference reports the test results concluding that the combined effect of reduced flow disruption and the "swirl jet", gave a reduction of 8.5 KPa (2.5 in-Hg) in the back pressure at high engine speeds.
However, it has been found that no significant ejector effect is likely to occur with all these annular ring type outlet designs because any tendency to reduce pressure by the re-entering bypassed flow is mostly negated by the main exhaust flow moving upstream, along the exhaust centerline, toward the low pressure region temporarily caused by the annular ring outlet designs. Thus, it has been found that the small amount of pressure reduction realized in the Aisin Seiki swirl jet design is attributable to reducing the disruption of the main exhaust flow caused when the bypassed flow is re-introduced into the main exhaust flow and that the annular ring type outlet designs do not produce a significant ejector effect.
It should be apparent from the above discussion and the prior art that efficient ejectors are very difficult to design. Although various designs have been proposed in the rior art, none of the prior art references claim to have obtained a pressure reduction significantly beyond that which is expected for minimizing the disruption effect. As discussed above, the swirl jets and the annular ring outlet designs were effective in reducing the disruption of the main exhaust flow thereby minimizing the increase in turbine exhaust pressure. However, these designs have been found to be inefficient in creating any significant ejector effect which can actually reduce turbine back pressure. In fact, none of the above noted prior art references disclose an effective ejector design that will actually reduce the turbine back pressure thereby increasing engine performance and efficiency.
Therefore, despite the progress made in the art in recognizing the potential benefits of an ejector effect in the re-entry of the bypassed flow, there is no known ejector design that can attain the ejector effect in a turbocharger. Thus, there exists an unfulfilled need for an ejector design for use in a turbocharger that is effective in reducing the turbine back pressure. Furthermore, there also exists an unfulfilled need for such an ejector design which is cost effective and easy to manufacture.